Process for producing high-capacity concrete beams or girders

ABSTRACT

A prescribed prestressing process is employed to construct precast concrete beams and girders. The process is utilized where an early concrete strength is too low for transfer of the full pre-tensioning force on a daily schedule to avert an otherwise serious and costly production delay. The process described provides the producer a reliable way of making beams and girders that are prestressed to take advantage of the higher concrete strength characteristics. The consequent economic advantage of higher structural capacity beams and girders is thereby realized. Additionally, beam or girder camber is controlled by the process, fostering production of a superior quality product.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This application relates to U.S. Provisional Patent Application No.61/280,109 filed on Oct. 29, 2009, entitled A METHOD FOR PRODUCINGHIGH-CAPACITY CONCRETE BEAMS, which is hereby incorporated herein in itsentirety by this reference.

FIELD OF THE INVENTION

The present invention relates generally to the prefabrication ofstructural building materials, and, more particularly, to theprefabrication of concrete beams or girders. Specifically, variousembodiments of the present invention provide an apparatus and process torealize economic and quality benefits by producing concrete beams orgirders of high structural capacity through practical steps that areeasily implemented in most precast beam/girder production plants.

BACKGROUND OF THE INVENTION

Progress in the production of concrete beams (also known as “girders”)for construction of bridges and buildings was greatly stimulated in the1950's when the technique of prestressing the concrete was proven tohave advantages in the United States. There are two known techniques ofprestressing: pre-tensioning and post-tensioning. Both techniques ofprestressing employ steel cables or bars to apply and hold a concretemember in compression. Prestressing can be referred to as “activereinforcement” as compared to “passive reinforcement”, such as isobtained with mild reinforcing steel (rebar).

Pre-tensioning is the predominate form of prestressing employed in theprecast concrete industry. This technique involves stretching steelcables with a high tensile force, with the cables held between fixedabutments that are situated at each end of a casting platform, or “bed”,and placing concrete in forms on the bed, which forms encase the cablesto form a beam or girder. At a later time, after the concrete has gainedsufficient strength and bonded well to the cables, the cables arereleased from the abutment anchors, the forms removed, and the completedbeam or girder is lifted from the bed.

It is of great economic importance that this procedure be completed on adaily cycle. In order to do this, heating the newly cast concrete at acuring temperature as high as 180 degrees F. to accelerate concretestrength gain has been a common practice.

The post-tensioning technique is not generally practiced in precastconcrete production. Post-tensioning is more expensive per pound thanpre-tensioning, so it has been employed sparingly in the production ofbeams or girders other than in special cases to meet designrequirements.

In recent years, there has been remarkable progress in making concretethat has much higher strength than ever before. Ultimate 56 daycompressive strength is now possible in the 10,000 psi to 20,000 psirange, which is up to 10,000 psi higher than strengths attainable ashort time ago.

However, there has not been an advantage taken of higher strengthconcrete by employing a proportionately higher prestressing force in thedesign of precast beams or girders. Beam or girder load carryingcapacity is increased dramatically when, using the same beam or girdersize and shape as those made with “standard” concrete, stronger highperformance concrete (HPC) is employed with a substantially greaterprestressing force. This fact was demonstrated on an experimental bridgeproject where HPC beams having a 56 day strength of 13,600 psi wereconstructed with approximately 60 percent more prestressing force thanstandard beams made with 6,000 psi concrete. Test results proved thatfour HPC beams had the same load carrying capacity as seven standardbeams for “twin” bridges of an identical span and roadway width.Although the cost per beam was higher for the HPC beams, the cost of thebridge superstructure having four high structural capacity beams wasapproximately 15% lower than the bridge having seven standard beams.This project confirmed the economic viability of employing higherstructural capacity beams made with superior concrete strength andconstructed with a high prestressing force. However, industry has notreaped the benefits of these features to achieve an improved and moreeconomic product. There are certain problems that must be solved.

In addition to the common precautions observed in the design of aconcrete mix, there are two important factors that must be dealt withconcerning concrete durability. Both of these factors pose potentialproblems in making durable concrete beams or girders, as well as otherconcrete members. The first is known as alkaline-silica reaction (ASR);the second is called delayed ettringite formation (DEF). ASR is causedin large part by high alkalinity in the concrete reacting over time withsilica in the aggregate. In severe cases, which are not uncommon, thisreaction results in cracking and destruction of the concrete.

On the other hand, it has been learned recently that DEF is promotedprincipally by curing the concrete at a very high temperature. DEFtypically occurs over time in mature concrete. It has been mistakenlyidentified as ASR in some cases, because its apparent failure mode issimilar to the failure mode attributable to ASR.

The solutions to both problems are now known. Damage due to ASR can beavoided by substituting another cementitious material such as fly ash orslag for a portion of the cement in the mix to reduce net alkalinity.The drawback to this approach is that early concrete strength gain isslowed. Although final strength is typically very high, the concretestrength required for transfer of stress (the “release strength”), isnot reached in time for daily recycling on the prestressing bed. Dailyrecycling of the bed is critical to a beam or girder manufacturer'seconomics.

DEF can be avoided by restricting concrete curing temperature to amaximum of approximately 160 degrees F. Here again, because earlyconcrete strength gain is dependent on curing temperature, the lowertemperature requirement makes attaining release strength overnight lesslikely.

Thus, there are two factors that have constrained production of superiorand more cost-effective beams or girders prefabricated with HPC. Sincehigher strength concrete beams or girders containing a high prestressingforce have been shown to produce a significant lower cost for acompleted structure, it is important to have a way of making prestressedHPC beams or girders on a daily production cycle.

Control of camber in concrete beams or girders can be yet anotherserious problem. Camber is the arching upward of a beam/girder or slabthat is prestressed when the prestressing force is located below thecentroid of the concrete. In almost all cases, the pre-tensioning forceapplied to a beam or girder on a pre-tensioning bed is well below thecentroid of the concrete. When a prestressing force (which is acompressive force) is applied to concrete, the concrete immediatelyshortens elastically as the force is applied. Thereafter, there is aninelastic shortening due to a phenomenon known as “creep” of theconcrete. The amount of creep is a function of time, the level ofcompressive stress, and the modulus of elasticity of the concrete.Camber takes place in a prestressed concrete beam or girder when theconcrete fibers in the lower portion of the member are under a highercompressive stress than the fibers in the upper portion. Creep of theconcrete continues to shorten the bottom of the member as time passes,causing camber to grow. There have been cases where camber growth hasbeen so great that beams or girders became unfit for use in structuresand were rejected. The economic implications of such a problem go wellbeyond loss of money by the precaster having to manufacture substitutebeams or girders. The construction company, depending upon timelydelivery of product for constructing the bridge or building, is impactedby delay that ensues while new beams or girders are manufactured toreplace the rejected ones.

One objective of the present invention is to provide a process that canbe readily implemented by beam or girder manufacturers to overcome theseproblems.

SUMMARY OF THE INVENTION

The various embodiments of the present invention provide a process formaking precast beams or girders that have a greater load carryingcapacity by employing a strategy that also provides additional controlof quality. The process described makes it practical to use higherstrength concrete that carries a high prestressing force. A substantialadvantage is obtained by the following combination of steps to achievesuperior load carrying capacity and quality and achieve advantageouseconomic results.

First, the full prestressing force required by the design requirementsfor a beam or girder is not introduced by pre-tensioning, as is nowroutinely done. Instead, only a portion of the full design force isapplied by pre-tensioning while the beam or girder is on a prestressingbed. A pre-tensioning force is applied that is at least a magnitude thatwill allow the beam or girder to be removed from the bed and withstandstresses experienced in handling and storage. The pre-tensioning forceneeded is readily calculated as a part of production procedures as iswell-known to persons skilled in the art.

The purpose of applying only a partial prestressing force is to allowearlier release of the pre-tensioning cables or rods, which release ismade possible because the prestressing force that is applied to theconcrete by pre-tensioning is reduced and thus permits the concretestrength to be lower before release of cables or rods from theabutments. Thus, the concrete beam or girder, although having an initiallower strength, can be removed from the bed earlier. Also, the effectsof low early concrete strength that is caused by adjusting the concretemix to diminish the prospect of ASR, and the lower curing temperature tocombat DEF, as well as other factors that result in a concrete strengthtoo low to carry the full prestressing force, are effectively managed,while daily cycling of the bed is achieved.

Second, after a beam or girder is removed from the bed, the beam orgirder is stored on supports near its ends, so that gravity acting onthe beam or girder counteracts most of the prestressing force and thusresists camber growth due to flexural stresses. The result is thatlittle, if any, inelastic concrete creep and camber growth occurs overtime.

Third, the remainder of the required prestressing force for the beam orgirder is induced by post-tensioning. Post-tensioning can beaccomplished at any time of the manufacturer's choosing, typically justseveral days before shipping the beam or girder to a customer's jobsite.By this timing strategy, unwanted camber growth can be eliminated.

An added operational advantage produced by this process is thatpost-tensioning is performed away from the casting area at a distancefrom the prestressing bed, and therefore it is not on a criticalproduction path because it does not affect the high intensity coreactivity of the beam or girder manufacturer. Also, because a range ofpost-tensioning forces can be applied, the manufacturer can potentiallybuild an inventory of partially constructed beams or girders and thussupply beams or girders to customers more quickly than if constructionof the beams or girders had not yet begun.

The foregoing and other objects, features, and advantages of the presentinvention will become more readily apparent from the following detaileddescription of various embodiments of the present invention, whichproceeds with reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

The various embodiments of the present invention will be described inconjunction with the accompanying figures of the drawing to facilitatean understanding of the present invention. In the figures, likereference numerals refer to like elements. In the drawing:

FIG. 1, comprising FIGS. 1A, 1B, and 1C, illustrates the basic processflow for beam or girder production in accordance with one embodiment ofthe present invention.

FIG. 2, comprising FIGS. 2A and 2B, shows an embodiment of a beam orgirder produced in accordance with the process of the inventionillustrated in FIG. 1.

FIG. 3 illustrates a cast-in-place concrete deck comprising the beam orgirder shown in FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1A, 1B, and 1C illustrate a flowchart depicting a non-limitingexample of the manufacturing process for a high strength concrete beamor girder, which synergistically allows for the use of high strengthconcrete combined with a rapid and economical cycling of themanufacturing bed, while providing a beam or girder strength that takesfull advantage of the high strength concrete.

The example process of FIG. 1 begins in a step or operation 100, asshown in FIG. 1A, and continues in an operation 102 in which themanufacturing bed is prepared for use. Then, in an operation 104,reinforcement and pre-tensioning strands, for example, cables or rods,are installed in place on the bed, along with post-tensioning ducts, andanchorages. Next, in an operation 106, tensile force is applied to thepre-tensioning strands that were deployed in operation 104. In anoperation 108, the elongation of the pre-tensioning strands is measuredand recorded. An operation 110 is then performed which assembles theforms in place on the bed to provide a structure that determines thebeam or girder shape.

In an operation 112, the high strength concrete is mixed. In certainexample embodiments, a high cementitious content is used, along withwater-reducing and plasticizing admixtures. The cementitious material incertain example embodiments comprises, for example, Portland Cement. Incertain example embodiments a portion of the cementitious materialcomprises an alkalinity reducing material such as, for example, fly ashor slag to prevent the alkalinity of the mixture from being too high.Otherwise, alkalinity can cause a very serious reaction with silica inthe aggregate, resulting in severe cracking of the concrete. While theintroduction of the alkalinity reducing material does not materiallyaffect the ultimate strength of the resultant concrete, the strength ofthe concrete is reduced in the short term, and other measures describedherein are preferably taken to compensate for the short term lack ofstrength and its impact on the manufacturing process. The result is asynergistic combination that provides a beam or girder that takes fulladvantage of the strength of the highly cementitious mixture, avoids asilica reaction, and yet allows a rapid cycling of the manufacturingbed.

In an operation 114 the concrete mixture is poured into the form. Aportion of the mixture is also poured into a number of cylindrical orcubical forms which allow the strength of the concrete to be sampled atvarious times. The curing apparatus is put in place in an operation 116.Then, in an operation 118 the concrete is cured.

As shown in FIG. 1B, the strength of the concrete is measured in anoperation 120, typically using one or more of the concrete cylinders orblocks mentioned in the description of operation 114. A decisionoperation 122 determines whether the concrete has achieved a strengththat is at least adequate to endure pre-tensioning (the “releasestrength”), removal from the bed, and storage. If it is determined thatthe strength of the concrete is not adequate, then a wait operation 124is performed to allow the concrete to gain more strength, and theprocess returns to operation 120. If it is determined in operation 122that the strength of the concrete has achieved a strength that is atleast adequate to endure pre-tensioning, removal from the bed, andstorage, then the process continues with operation 126 which releasesthe pre-tensioning strands from their abutment anchorages, thus placingthe beam or girder under compression.

Next, in an operation 128, the curing apparatus is removed to allowaccess to the beam or girder. Thereafter, in an operation 130 the formsare removed and cleaned for reuse. Then, in an operation 132, the beamor girder is moved to storage. In certain example embodiments, the beamor girder is placed on supports proximate to the beam's or girder'srespective ends, which allows the beam or girder to avoid camber growth,since the force applied in operation 106 is of less than full magnitude.The beam or girder, having gained sufficient strength to support its ownweight and avoid deflection can be stored indefinitely. This removal ofthe beam or girder from the bed permits the bed to be re-used, andallows the beam or girder to gain strength over a period of time instorage. The timing of the removal from the bed is earlier than wouldotherwise be possible, and this early removal allows the bed to be usedagain for making another beam or girder. The removal of the beam orgirder from the bed can be performed when the high strength concrete isrelatively weak, because the pre-tensioning strands that were releasedin operation 126 have imparted only a portion of the total eventualprestressing force, yet a sufficient force for removing the beam orgirder from the bed. The pre-tensioning in operation 126, which impartsonly a portion of the full prestressing force, thus synergisticallyallows high strength materials to be used even though those materialsare relatively weak on the day after casting.

As described above, in the operation 132 the beam or girder is moved tostorage and placed, for example, on supports proximate to the ends ofthe beam or girder in order to limit camber growth. An operation 134shown in FIG. 1C is performed wherein the beam or girder is kept instorage while it gains strength sufficient for the full prestressingforce. The amount of storage time can vary dependent on the formulationof the materials of the concrete, and also can vary with strengthrequirements for the beam or girder. The beam or girder can be allowedto gain strength over any desired amount of time in order to takeadvantage of the strength potential of the materials used, or meet timeconstraints that call for beams or girders of lesser strength in arelatively short amount of time. Next, in an operation 136 apost-tensioning force is applied. Then, in an operation 138 cement groutis injected into the tendon ducts employed in post-tensioning. Next, inan operation 140 the grout is allowed to cure over a period of time.Finally, the process is concluded in an operation 142.

One example embodiment of the beam or girder that is the product of theprocess described in conjunction with FIG. 1 is shown in FIG. 2. Asshown in FIG. 2A, a beam or girder 200 comprises prestressed highstrength concrete. The beam or girder is cast on a manufacturing bed(not shown) using a set of forms which determine the shape of the beamor girder. The example beam or girder shown in FIG. 2A has a resultingshape generally referred to as an “I-beam.”

As shown in FIG. 2A, the beam or girder 200 is prestressed duringinitial manufacture of the beam or girder on the bed usingpre-tensioning strands 202 described earlier in conjunction withoperations 104 and 106 illustrated in FIG. 1A. The strands 202 arepreferably installed on the manufacturing bed prior to the erection ofthe forms used to contain the high strength concrete. Semi-flexiblepost-tensioning ducts 204 are also installed as described earlier inconjunction with operation 104 illustrated in FIG. 1 A. Thepost-tensioning ducts 204 terminate at post-tensioning anchorages thatmay be installed employing reusable blockout forms 206, as shown in FIG.2B. As shown in FIG. 2B, there may be one or more post-tensioning ducts204 which are placed into an approximate parabolic curve. Tensile forceis then applied to post-tensioning strands inserted through thepost-tensioning ducts 204 in operation 136 described earlier to providethe remainder of the required prestressing force for the beam or girder200.

In accordance with another aspect of the present invention, a beam orgirder having sufficient area at the beam or girder ends foraccommodating post-tensioning tendons that pass through more than onebeam or girder is provided to connect with another beam or girder whichis aligned with the first and is located in an adjacent span to form acontinuous structural frame. A continuous frame, in which two or morespans are connected, reduces structure cost and makes longer spanspossible. Precast beams and girders that are connected bypost-tensioning tendons at support points such as piers or columns tomake a continuous frame require an area at the beams' or girders' endsto permit “through” tendons to connect adjacent spans. If the area atbeam or girder ends is not available due to the presence ofpost-tensioning anchorages previously placed at the ends of girders in“end blocks”, as is the present practice, there is insufficient room forthe through tendons to pass through to make the connection. Thedescribed beam or girder shape permits locating previously placed tendonanchorages at a distance away from beam or girder ends, thus creatingroom for tendons to pass through to make a continuous frame.

In accordance with another aspect of the present invention, a pluralityof beams or girders 200 can be deployed to construct a cast-in-placeconcrete deck 300, as shown in FIG. 3. The deck 300 comprises at leasttwo beams or girders 200. The spacing between adjacent beams or girders200 varies according to loading and length of a span to a maximumspacing, for example, 15 feet. Additionally, the deck 300 comprises oneor more deck panels 302. For example, each deck panel 302 may be afour-inch thick prestressed concrete slab. Also, each deck panel 302 mayfurther comprise a continuous neoprene strip 304 at each end of the deckpanel in contact with the beams or girders 200 that support the deckpanel. Additionally, the outside beam or girder 200 at each edge of thedeck 300 is provided with a flange 210 that is preferably precast withthe beam or girder. The flange 210 completes the concrete form for thedeck 300 and thus retains concrete poured to construct the deck 300, aswell as supports a finishing machine (not shown) employed to smooth thesurface of concrete poured to complete the deck. As shown in FIG. 3, theflange 210 may also be subsequently employed to support an attachedbarrier rail or curb 306 of the deck 300 installed at the edge(s) of thedeck. The modular elements shown in FIG. 3 enable a bridgesuperstructure to be built quickly with high quality at low cost. Byfabricating beams or girders 200 of higher concrete strength than in thepast and using a commensurately higher prestressing force to producegreater structural capacities, significant economy is achieved byrequiring fewer beams or girders for a given span and by the eliminationof overhang forms and most on-site superstructure formwork by employingthe modular elements shown in FIG. 3.

While the foregoing description has been with reference to particularembodiments and contemplated alternative embodiments of the presentinvention, it will be readily appreciated by those skilled in the artthat changes in these embodiments may be made without departing from theprinciples and spirit of the invention.

1. An improved process using high strength concrete and employing asubstantially greater prestressing force to fabricate a concrete beam orgirder having a significantly higher structural capacity, theimprovement comprising the steps of a) applying only a portion of apredetermined prestressing force by pre-tensioning while the beam orgirder is on a prestressing bed and b) applying the remainder of thepredetermined prestressing force to the beam or girder bypost-tensioning at a later selectable time, thereby avoiding unwantedcamber growth and whereby a high structural capacity beam or girder isproduced that reduces the number of beams or girders of lesserstructural capacity required in a given structure, thus offeringsubstantial cost savings.
 2. The process of claim 1 wherein step a)applies a pre-tensioning force that is at least a magnitude that enablesthe beam or girder to be removed from the prestressing bed andwithstands stresses experienced in handling during removal, therebyallowing early removal of a partially prestressed beam or girder from aform on the prestressing bed and thus permitting subsequent fabricationof a next beam or girder without substantial delay.
 3. The process ofclaim 1 wherein step a) applies a partial pre-tensioning force to thebeam or girder, the partial pre-tensioning force being adequate, at aminimum, to withstand handling and hauling stresses, thereby enablingpractical fabrication and stockpiling of beams or girders for emergencyor other future uses over the course of a prolonged period of time. 4.The process of claim 2, further comprising the steps of c) removing thebeam or girder from the form on the prestressing bed after step a) andd) storing the beam or girder on supports near respective ends of thebeam or girder, whereby gravity acting on the beam or girder counteractsthe prestressing force and thus resists camber growth due to flexuralstresses so that inelastic concrete creep and camber growth are reduced.5. The process of claim 1 wherein step a) applies a pre-tensioning forcehaving a selectable magnitude to control a horizontal trajectory of thebeam or girder, the pre-tensioning force being a magnitude less than apre-tensioning force that would cause the beam or girder to bowsignificantly, thereby avoiding unwanted camber in a cost-effectivemanner.
 6. The process of claim 1 wherein step a) applies apre-tensioning force having a selectable magnitude, the pre-tensioningforce being a magnitude adequate to produce an internal bending momentin the beam or girder that balances an external self-weight bendingmoment of the beam or girder, whereby the balance of internal andexternal bending moments negates beam or girder camber growth, therebyenabling practical storage of beams or girders for deployment at anindefinite future time. 7.-12. (canceled)